Degenerative disc disease, spinal trauma and tumors are common and painful conditions suffered by a significant portion of the population. In some instances, the pain and complications caused by these conditions may be bad enough to require that one or more vertebra, facet joints, and/or intervertebral discs be removed from the spinal column. In these instances, arthrodesis or bone fusion are common treatments used to facilitate the realignment and/or fixation of the spinal elements. Typically, two types of assemblies are known for securing one or more vertebra in order to obtain arthrodesis or bone fusion. The first type of assembly generally includes two posterior vertebral plates disposed longitudinally on either side of the spinous processes. Each plate is attached between adjacent vertebra using bone anchoring elements, such as bone screws. Together the plates provide a rigid vertebral fixation. The second type of stabilizing assembly generally includes two posterior vertebral rods disposed longitudinally on either side of the spinous processes. Like the plates, these rods are attached between adjacent vertebra using appropriate bone anchoring elements to provide a rigid vertebral fixation.
One drawback of rigid fixation derives from the fact that significant loads are placed on the stabilizing assemblies and particularly on the anchoring sites thereof. These loads may result in the loosening of the assembly from the vertebra or even the breaking of the assembly. The stabilizing assemblies are often supplemented with bone grafts formed from transplanted bone tissue and/or artificial fusion cages in order to fuse the adjacent vertebra. Unfortunately, such bone grafts and fusion can cause serious complications throughout the patient's life because fusing the vertebra subjects the remaining spinal elements to high stress and degeneration. This is particularly true of the remaining adjacent vertebra and vertebral discs because these elements must accommodate an even greater degree of motion. Moreover, spinal fusions limit the range of motion for patients in flexion, extension, rotation and lateral bending.
Similar assemblies are also used to correct spinal deformities, such as those associated with scoliosis, spinal traumas and tumors. In these systems correcting rods spanning two or more vertebra are typically implanted on either side of the spinous processes.
In addition to rigid spinal stabilization assemblies, some semi-rigid devices have been proposed. Some such devices are aimed at preserving a small amount of intervertebral elasticity in order to assist in subsequent bone fusion and to reduce stress. Other such devices provide stabilizing rods that are capable of sliding in a vertical direction with respect to the vertebra in order to accommodate spinal column growth.
Because of the problems associated with rigid vertebral fixation, the use of artificial disc prostheses has become an attractive option to many patients. These disc prostheses may be inserted in place of a natural vertebral disc in order to simulate at least some of the natural intervertebral movement and to restore proper disc height. Ideally, a disc prosthesis will operate in conjunction with the facet joints to restore the full range of motion of the spine. The facet joints are posterior vertebral elements that help to support axial, torsional and shear loads that act on the spinal column. When a facet joint becomes diseased or deformed (e.g. enlarged), it is sometimes necessary to remove part or all of the facet joint in a full or partial facetectomy. Unfortunately, the removal of the facet joint may destabilize the spinal column by decreasing the stiffness in flexion, extension, lateral bending and rotation. Moderate or advanced facet disease and facetectomies, or other prior destabilizing procedures, may be a contraindication for prosthetic discs.
Thus, a need exists for a spinal stabilization system that preserves at least some of the physiological mobility of the vertebra, vertebral discs, and facet joints after a prior destabilizing procedure, such as a facetectomy or laminectomy.